1. Field of the Invention
The present invention relates to packet-based networks, such as a wireless telephone system having packet-based connections (e.g., Ethernet facilities) between its base stations and central offices.
2. Description of the Related Art
Traditionally, base stations of wireless telephone systems are connected to central offices via terrestrial (electrical or optical) links that transmit DS1 or E1 synchronous, time-division multiplexed (TDM) signals or synchronous optical network (SONET) signals. DS1 signals can provide data rates up to about 1.544 Mbps, while E1 signals can provide data rates up to about 2.048 Mbps.
A base station transmits, to one or more wireless units (e.g., mobile cell phones) located within the base station's cell site, wireless signals at specific frequencies, whose accuracies are typically required to be within 50 parts per billion (PPB) (for Third Generation Partnership Project (3GPP) standards) or 16 PPB (for Open Base Station Architecture Initiative (OBSAI) standards). In order to achieve these high accuracies, base stations can be implemented with global positioning system (GPS) receivers that are capable to recovering clock signals with accuracies better than 16 PPB. Unfortunately, it can be prohibitively expensive to provision each base station in a wireless telephone system with such a GPS receiver.
Another possible solution would be to provide each base station with its own free-running oscillator capable of generating a clock signal of sufficient accuracy, but this too can be prohibitively expensive.
Yet another known technique for providing a sufficiently accurate clock signal is for the base station to use line timing to derive its reference clock signal from the received synchronous signals. In conventional line timing at a base station, different clock signals are recovered from two or more different incoming synchronous (e.g., DS1/E1) signals received from one or more central offices, and one of the recovered clock signals is selected for use in (1) generating one or more outgoing synchronous (e.g., DS1/E1) signals for transmission back to the one or more central offices and (2) generating the base station's wireless signals for transmission to the base station's associated wireless units.
In order to provide higher data rates (e.g., 100 Mbps or higher) between base stations and central offices, wireless telephone systems are being proposed to employ packet-based Ethernet connections (i.e., facilities) between base stations and central offices, instead of the conventional DS1/E1 connections. As used herein, the term “Ethernet” refers to technology conforming to local area network (LAN) standard IEEE 802.3 (1980) and/or to any of its extensions. Conventional Ethernet facilities between two nodes rely on either (1) each node having its own local oscillator for use in generating the reference clock signal used to control its Ethernet transmissions to the other node or (2) loop timing, wherein one node's physical interface (PHY) derives its reference clock signal (which that node uses to control its Ethernet transmissions to the other node) from the Ethernet transmissions received from the other node, which other node has a local oscillator for use in generating its reference clock signal. Existing Ethernet standards dictate clock signal accuracies to within only about 100 parts per million (PPM). As such, reference clocks for traditional Ethernet technology are not sufficiently accurate to use as references for generating the wireless signals transmitted by the base stations of a conventional wireless telephone system.